This invention concerns thermoforming machines of the type in which thermoplastic sheets are first heated in an oven and then transferred to a forming station where a heated sheet is drawn onto tooling forms by vacuum and/or air pressure to be formed into an article.
In some instances, a clamping frame is used to hold thinner sheets which would otherwise become too weak when heated to be transferred between stations in the machine.
When using a clamping frame, a continuous in-line process is not possible as the clamping frames must be returned to the first station and reloaded with a cut sheet.
Rotary transfer thermoforming machines are known such as described in U.S. Pat. No. 3,925,140 issued on Dec. 9, 1975, which use continuous rotation of a carrier member to carry the frames through the station and to return the frames to a load/unload station after successive rotation through the heating and forming stations.
For very large articles, rotary transfer thermoforming machines occupy excessive plant floor space.
An in-line shuttle transfer has also been used, but this either is too slow if a single clamping frame is used, or requires two molding stations on either side of an oven.
Another problem is encountered in thermoforming parts with substantially angled deep portions, as excessive stretching of the material would occur if the sheet is simply formed onto the tool shape.
Draping techniques are sometimes employed for closed shapes forming dished containers, with vacuum (or air pressure) causing a pronounced sagging of the sheet material preparatory to the forming step. Such technique is not useable with a nonclosed shape or with angled ends connected with a flat center section.
U.S. Pat. No. 4,744,848 issued on May 17, 1988, describes a frame with hinged side portions which floats to allow forming with minimal stretching. This arrangement relies on engagement with a male and female mold to cause hinging of the frame ends which in turn bends the sheets into closer conformity with the mold contours prior to forming. See also U.S. Pat. No. 3,574,807 issued on Apr. 13, 1971, which describes a frame hinged in the middle to form an angled article shape.
These patents do not disclose an adaptation of a hinged clamping frame to automated thermoforming apparatus.
It is an object of the present invention to provide a frame having a section pivot capability for sharply angled elongated parts which is adapted to an automated system and which does not require engagement with a male and female contour to be actuated.
These objects and others which will become apparent upon a reading of the following specification and claims are accomplished by a linear transfer system for a set of two clamping frames.
The clamping frames are each successively moved through a load station, heating or oven station, a forming station, and an elevator/return station by a reciprocated transfer bar structure. Each clamping frame is slightly elevated in the heating, forming, and elevator/return stations to allow retraction of the transfer bar structure to receive a trailing clamping frame for separate advance.
Each clamping frame is elevated to a height above the heating station oven, and transferred back over the top of the oven, to a position above the loading station. After partial lowering, a cut sheet is loaded into the frame, which is thereafter lowered onto the transfer bar structure.
Each clamping frame has pivoted end sections which are each engaged by pairs of extensible gear posts when each frame is in the forming station, the posts each advancing a cam roller into and out of engagement with one of a pair of side channels on either side of the frame by operation of a drive system. The pivoting of the end sections brings the clamped sheet into closer conformity to the mold surfaces. The gear posts are retracted to disengage the cam rollers with the side channels to allow advance of the clamping frame to the elevator/return station.
An array of pneumatic cylinders and a series of clamping bars are arranged around the inner perimeter of the clamping frame. The cylinders are operated to retract or advance the clamping bars by application of pneumatic pressure communicated through manifold bars engaging one of the lift plates protruding from each end of the clamping frame.
The manifold plates are mounted on elevating cylinders located at the load and form stations so as to establish a fluid connection to the cylinders when the clamping frame is raised off the transfer bars.
In the load station a cut sheet is raised within the clamping frame, and the cylinders operated to clamp the cut sheet in the clamping frame.
In the forming station, the cylinders are retracted as the sheet is formed to release the sheet and allow removal of the formed article.
This invention allows a very large sharply angled article to be efficiently formed in an automated system by pivoting of the clamping frame end sections in the forming station by a disengageable drive.